We're planning a Pivotal Tracker upgrade on Jan 23. As part of this release, we will be introducing a new API version (V3), which will make it easier to follow project activity, allow you to add file attachments, move (re-prioritize) stories, associate source commits with stories, and more.
The current API version (V2) will not change, but V1 will no longer work. If you're still using V1, you will need to change your client code to use V2 or V3.
To find out what's changing in V3, continue reading.